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Abstract

Alpha diversity, population dynamics and interactions of ant assemblages were
studied in a pecan orchard located in Mumford, Texas. The assemblages included the
invasive species Solenopsis invicta Buren, known as the red imported fire ant (RIFA).
The study addressed the major question of what is the response of the ant assemblage to
the reduction of RIFA following insecticide applications (bait and contact insecticide)
and the impact of these on individual species. To address this question three treatments
were established in a 16 hectare area in the orchard. The treatments were randomly
assigned in 1.33 hectare blocks with four replications and periodically monitored.
Treatments were: 1) insect growth regulator (IGR) bait treatment (ExtinguishTM, active
compound is 0.5% s-methoprene) applied twice in 2000 and once in 2001; 2) the contact
insecticide chlorpyrifos (LorsbanTM) applied on tree trunks four times in 2000 and once
in 2001; and 3) untreated Control. Blocks were sampled using pitfall traps, baited vials,
direct sampling, and colony counts. Data were analyzed by using ANOVA-GLM with
the LSD multiple comparison test to compare the effect of treatment on the ant
assemblage (using the Shannon index) and the effect on individual species. Additionally,
data obtained from Control plots were used to compare sampling techniques and to
determine what method is most efficient for collecting ants in this agroecosystem.
Shannon indices were estimated for each method and compared. The ant assemblage
consisted of 16 ant species. S. invicta was the most abundant followed by Paratrechina
sp. and Monomorium minimum. The IGR treatment consistently reduced RIFA (77%).
Native ants were found to coexist with RIFA in the Control and chlorpyrifos plots at
lower densities and maintained higher densities in IGR plots. Chlorpyrifos trunk
treatment did not have a significant impact on RIFA or native ant densities. The native
ant, Dorymyrmex flavus, was greater in IGR plots following RIFA reduction and higher
densities were found to persist for more than two years after the last IGR treatment.
During this period, D. flavus was observed carrying large numbers of dead RIFA, some
taken inside the nest, and some disarticulated RIFA taken out of the nest. RIFA remains
were accumulated in D. flavus middens, further indication of an important interaction
between these two species. These results indicate D. flavus resisted reinvasion by RIFA.